The present invention relates to an optical transmission system using a wavelength division multiplexing technology.
In recent years, high-dense wavelength division-multiplexing systems have been realized with the advance of the laser wavelength control technique. In the ITU-T Recommendation G.692, it is standardized to arrange laser wavelengths at wavelength intervals of 100 GHz to 1000 GHz corresponding to multi-channels.
That laser wavelength control is usually controlled under temperature adjustment. However, in some passbands for wavelength division multiplexing, the temperature is adjusted with a precision of, e.g. +−0.5° C. or less. In some cases, predetermined stable wavelengths can be obtained using the wavelength locker.
The wavelength division-multiplexing transmission in which the wavelength interval is widened up to several THz has been proposed to eliminate the temperature control.
However, narrowing densely the wavelength interval requires adjusting the temperature of a laser with high precision. The laser current flows to control the optical output power, thus varying the wavelength. This results in more complicated control and in increased costs of the optical transmitters.
In the wavelength division-multiplexing system, the optical transmitters are arranged corresponding to the number of wavelengths to be multiplexed while the optical receivers are arranged corresponding to the number of wavelengths to be multiplexed. Hence the problem is that the costs of each optical transmitter or each optical receiver must be reduced to decrease the costs of the entire system. This is particularly important to applications that require inexpensive systems even in low multiplicity.
When the wavelength interval is widened up to, for example, 2.5 THz (20 nm), some systems can neglect the laser temperature adjustment. However, to multiplex, for example, four waves, a wide range of about 60 nm is required so that the lasers must be arranged over a wide range.
Moreover, there is the problem in that because the wavelength interval exceeds the band of the C band (1530 nm to 1560 nm) Erbium-doped fiber amplifier (EDFA), the loss margin cannot be ensured in some cases.
With no temperature control, it is difficult to use an external modulator integrated laser beam source, in consideration of the temperature characteristics of the modulator. It is required to modulate directly a semiconductor laser with good temperature characteristics. Consequently, the chirping in the laser direct modulation of the semiconductor laser becomes larger, compared with the use of the external modulator, so that the transmission distance is limited.